With increased use and decreasing availability of petroleum supplies, gasification technologies of economical solid hydrocarbon sources such as, but not limited to coal, petrochemical coke, and solid biomass are currently becoming more attractive technically and economically as a versatile and clean way to produce electricity, hydrogen, and other high quality transportation fuels, as well as convert these solids into high-value chemicals to meet specific market needs. Currently there are abundant worldwide supplies of coal as well as a large market supply of petrochemical coke in the U.S. market. The vast majority of these supplies are utilized as fuel in coal-fired electrical plants in the United States or are shipped oversees as low cost fuels for foreign electrical generation.
However, with current gasification technologies, these solid hydrocarbon fuel sources can be used to produce significantly more attractive liquid fuels products, such as gasolines and diesel fuels, through the partial-oxidation of these solid hydrocarbon fuels in a gasifier to produce a syngas product. These solid hydrocarbon feeds, such as coal, petrochemical coke, and/or solid biomass, contain hydrogen and carbon, and can be partially oxidized at elevated temperatures in the presence of an oxidizing gas or vapor, such as air, oxygen, and/or steam to produce a “syngas” product. The chemistry for producing a syngas from hydrocarbon sources is well known in the industry and appropriate feeds and operating conditions can be selected to optimize the chemical reactions in producing the syngas.
The produced syngas is preferably comprised of hydrogen (H2) and carbon monoxide (CO). This syngas can then be converted into valuable liquid transportation fuels, such as gasoline and diesel, through various catalytic reforming processes. The most common and well-known of these processes is the Fisher-Tropsch process which was developed by German researchers in the 1920's. In a Fisher-Tropsch process, the syngas is reformed in the presence of a catalyst, typically comprised of iron and/or cobalt, wherein the syngas is converted into chained hydrocarbon molecules. The following formula illustrates the basic chemical process involved in the Fisher-Tropsch reaction:(2n+1)H2+nCO→CnH(2n+2)+nH2O  [1]
In conversion processes for the production of transportation fuels, the conditions are generally optimized to maximize conversion of the reaction products to higher molecular weight hydrocarbon compounds with carbon contents of about 8 to about 20 atoms. As with the syngas production process described above, various chemical processes for the conversion of syngas into liquid hydrocarbon transportation fuels are well known in the art.
Other processes include the conversion of solid hydrocarbon feeds, such as coal, petrochemical coke, and/or solid biomass into syngas (predominantly hydrogen and carbon monoxide) for use as a “clean fuel” in electrical production. The syngas produced by the process retains a relatively high BTU value as compared to the solid hydrocarbon feeds from which it is derived. Especially problematic for clean fuel production can be solid hydrocarbon feeds that are fossil fuel based (such as coal and petroleum coke), as these feeds may contain a significant amount of contaminants such as sulfur and/or nitrogen. These contaminants can be damaging to power generating equipment as well as pose environmental emissions impacts on commercial processes. By first gasifying the solid hydrocarbon fuels, these contaminants can be gasified and more easily removed prior to be using as a gas fuel for power generation. These “clean” fuels can then be used as a combustion fuel for high speed gas turbines or for producing steam for steam driven turbines in the industrial production of electrical power.
The benefit of using these solid hydrocarbon fuel sources is that they are economic fuels relative to liquid or gas hydrocarbon fuels. This is due in part to their low marketability for use as transportation or home heating fuels. This is also due in part to the often significant contaminants (such as sulfur and nitrogen) that are not easily removed from the solid fuel source, often relenting their use to commercial operations which can remove these contaminants as part of the integrated industrial processes.
However, many of the difficulties in using these solid fuels in conventional gasifier systems is in the existence of problems associated with “flowing” a solid fuel into a gasifier system. As gasification technology improves, it is critical that these gasifier systems be increased in size and capacity as well as become more efficient and produce an improved syngas product composition. However, as gasification systems increase in size, usually with a resultant increase in the number of burners per unit, the problems associated with inadequate feed systems and the inability to properly control the solids feed distribution between the multiple burners increase exponentially. These gasification systems operate at very high temperature often in the range of about 2000 to about 5500° F. Even small variations in feed supply rates to the burners can result in off-specification syngas products as well as damage to the equipment due to uneven heating. Uneven supply rates of the solids feed between the associated burners can also result in dangerous backflow conditions. Intermediate feed supplies can also result in isolated over-combustion (or localized explosions) which also can result in significant equipment damage or a shutdown of the gasification processes.
In the prior art, problems associated with solid feed systems were addressed in one manner by the use of “aerating” or “fluidizing” gases to fluidize the solids feed beds. Examples of this technology are illustrated in U.S. Pat. Nos. 4,338,187; 4,830,545; and 5,106,240. The problem with these devices is that a large amount of fluidization gases are used to enable the fluidization of the particulate bed. A significant problem exists in these processes in that the significant amount of fluidizing gas utilized has to be expelled through the burner and reaction chamber of the gasifier. The high volume of these fluidizing gases reduces the available capacity of the gasification unit due to the large amount of fluidization gases traveling through the system. Additionally, since these transport gases typically need to be non-oxidizing gases for safety reasons, these gases cannot be utilized in the reaction process for the production of syngas and thus are basically “contaminants” in the process. This additionally requires that the fluidizing gas contaminant must be removed from the final syngas product before it can be utilized. An additional problem with the use of fluidizing gases is that for proper operation of gasifier system, the fluidizing gas must be heated thus requiring a significant amount of the energy expended in the syngas production process. This energy must be removed from these fluidizing gases at a later stage in the process at considerably lower temperatures. This results in significant overall energy losses in the gasification processes.
Other proposed gasifier feed systems, such as illustrated in U.S. Patent Publication No. 2006/0242907 A1, integrate elaborate feed splitting systems such as the two-stage feed splitting system shown in the patent application. These systems require very elaborate construction and machining as well as are difficult to maintenance. In addition, the allowable voidage of the fuel feed to the system must be kept high in order to allow the feed to evenly flow through the different nozzles and stages, and prevent significant distributor/nozzle plugging. In essence, for these systems to work properly, the feed to the splitters must be effectively “fluidized” in order to prevent significant flow deviation and/or pluggage of the system. The net result is that this system results in the need of a “fluidization” gas to operate properly accompanied with the corresponding drawbacks as described in the systems prior.
What is needed in the industry is a simplified solid hydrocarbon fuels delivery system that does not require the solid fuels to be “fluidized”, and simplifies construction of the overall system, while maintaining reliability and improving overall performance and fuel capacity.